The present invention relates to a process for producing read only type optical disks, such as CD-ROM, and also write once type and rewritable type optical disks. In more particular, it relates to a process for producing optical disks excellent in attaining the speedup of replication process.
Optical disks may be roughly divided into read only type ones and writable ones. The former include laser disks (LD) and compact disks (CD) and are currently produced on a large scale. Read only type optical disks include, besides those mentioned above, CD-ROM, CD-I, DV-I and the like as multimedia means, and these are expected to grow greatly in the future. In particular, great expectation is harbored toward the use of optical disks as electronic publishing. As to the dimension of optical disks, small-sized disks 5 to 1 inch in diameter are becoming the mainstream. In the replication of such disks on a large scale, the electroplating process and the injection process, which constitute the mainstream of the processes for producing conventional LD and CD, have their limits in production speed.
In the prior art process, firstly, too much time is necessary because the stampers used are prepared by the electroplating process using nickel. Further, preparation of a large number of nickel stampers, which needs the application of electroplating process for each individual stamper, requires a still longer time. That is, it takes a long time from the receipt of the original tape to the completion of stampers, so that the process can hardly meet the requirement for a short delivery time as in electronic publishing. On the other hand, the injection method, used as a replication process, requires about 10 seconds per a disk and cannot be called a rapid replication. In this process, molten resin is filled in a mold provided with a nickel stamper at high temperature and high pressure, then cooled and taken out. A time of about 10 seconds is necessary to cool the resin at 330.degree.-350.degree. down to about 80.degree. C. In the case of CD-ROM, a reflecting layer of aluminum and the like is formed subsequently to the replication process, but the film-forming process is of a sufficiently high speed in itself. Therefore, the replication process is rate-determining in the whole production process.
Inventions regarding the speedup of a replication process include those disclosed in Japanese Patent Kokai (Laid-open) Nos. 1-115621 and 2-44540. The disclosed methods comprise pressing a nickel stamper having a surface unevenness pattern of information against the surface of a previously heated resin sheet to effect replication. They, however, are said to have a problem in the replicability of minute patterns of submicron order and, when the trend toward more minute patterns in future is taken into consideration, cannot be said an excellent process. As to the inventions regarding rapid replication based on a replication method using UV curable resins, namely so-called 2P (photo-polymerization) method, mention may be made of those disclosed in Japanese Patent Kokai (Laid-open) Nos. 56-51031, 56-87203, 57-50304, 63-153745 and 64-48246. The methods disclosed in these comprise coating a UV curable resin on a continuously moving transparent resin sheet, irradiating an ultraviolet light through the sheet to cure the UV curable resin while pressing a stamper against the sheet, then peeling the UV cured resin off the stamper to complete replication, thereafter forming a light reflecting layer and a protective layer and finally punching the sheet into the form of disk. These disclosed methods include one which uses a flat stamper, one which sticks one or more stampers onto the surface of a drum and rotates the drum as in a rotary press, or further one which uses plural stampers to permit a number of replications by one time irradiation. These disclosures, however, either describe nothing of the detail of the stampers to be used or only describe the use of nickel stampers prepared by conventional methods. When nickel stampers are made by conventional methods, a large number of stampers cannot be prepared in a short time and hence a large quantity of optical disks cannot be produced in a short period of time.
As to the inventions regarding stampers, on the other hand, those disclosed in Japanese Patent Kokai (Laid-open) No. 59-224320 and Japanese Patent Kokoku (Post-Exam. Publication) No. 3-46894 can be referred to. The disclosed methods comprise irradiating laser beams light-modulated according to the information to be recorded onto a photoresist layer formed on a silicon substrate (this process being called "cutting process"), followed by developing and etching to form a surface unevenness pattern, and using the resulting product as a stamper. In this case also, the preparation of a large number of stampers requires too much time because each stamper requires a cutting process each time for its preparation. That is, though the time efficiency is improved in the method because of the disuse of an electroplating process, the method requires, each time a stamper is produced, a cutting process which takes 60-30 minutes per one time. In this method, further, the shape and dimension of raised or sunken pits on the stamper formed, which represent the information, are apt to fluctuate among individual stampers. In the prior method, therefore, among the original molds obtained by patterning through the cutting process, only those which have passed strict inspection are to be used in the next process; this makes it difficult to prepare a large number of stampers by such methods. There is also known a method which prepares a nickel stamper directly in a short time by means of ion milling. This method, however, produces a stamper of a sunken pattern wherein the sunken parts and the raised parts of the pattern are reversed. When such a stamper is used to replicate optical disks by the above-mentioned injection method, minute patterns can be difficultly replicated, resulting in a large proportion of defective products.